The Structure and Properties of an Intermediate-Velocity Cloud in the Direction of the Globular Cluster M13
Abstract
A study of an intermediate-velocity cloud (IVC; VLSR ≈ -70 km s-1) detected in the foreground of the globular cluster M13 has been made using H I spectra obtained with the Lovell Telescope, Jodrell Bank (FWHP beamwidth 12') and H I spectral maps derived from Dominion Radio Astrophysical Observatory Synthesis Telescope observations (FWHP synthesised beamwidth ≈2'.8 x 2.'0). The radio data complement the optical absorption line spectroscopy of M13 cluster stars reported by Bates et al. The H I data obtained with the Lovell Telescope show extended emission from a broad-velocity component (FWHM 30 km s-1) of H I column density≥ 7 x 1O18. Two cloudlets are revealed in the H I observations lying on either side of M13 and having a two-component structure; the broad component (FWHM ≈ 32 km s-1) has an H I column density of 5 x 1O19 cm-2, while the narrow component (FWHM ≈ 8 km s-1) has a column density of ≈6 x 1O18 cm-2 as seen in the Synthesis Telescope observations. The cloudlets have an angular size ≈2' and are only partially resolved in the synthesis telescope data. Fine-scale gas structure is also evident from the cluster star spectroscopy with variations in Na I column density of a factor of ≥10 observed across the several arcminutes of the cluster face; there is also evidence for structure on an angular scale of several arcseconds. A column density ratio Na I/H I≈ 4 × 10-8 is measured for a cloudlet in the foreground of the cluster near the strongest Na I detection. Estimates are made of cloudlet gas density that are comparable with those reported for high-velocity cloud (HVC) concentrations. However, the present observations emphasize further the need for higher spatial resolution H I studies to provide firmer estimates of cloud properties.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 1996
- DOI:
- 10.1086/178197
- Bibcode:
- 1996ApJ...473..849S
- Keywords:
-
- GALAXY: GLOBULAR CLUSTERS: INDIVIDUAL MESSIER NUMBER: M13;
- ISM: ABUNDANCES;
- ISM: STRUCTURE;
- ISM: CLOUDS;
- RADIO LINES: ISM